Reader or transmitter and/or receiver comprising a shielded antenna

ABSTRACT

The reader or transmitter and/or receiver ( 22 ) includes a sheathed antenna formed of two groups of turns or coils ( 6, 8, 9; 16, 17 ). The first group of turns or coils forms a sheathed antenna. The second group includes two coils ( 16, 17 ) arranged between the central coil ( 6 ) and respectively the two end coils ( 8, 9 ) of the first group. The second group of coils is powered in phase quadrature relative to the first group of coils, the set of coils being arranged so as to obtain a magnetic field (H) whose amplitude (A) is approximately constant along the central axis ( 24 ) of the antenna inside the overall inner volume of the latter.

This application is a 371 PCT/EP02/12195 Oct. 31, 2002.

The present invention concerns a reader or a transmitter and/or areceiver fitted with a shielded antenna. In particular, the inventionconcerns a device of this type provided for communicating withtransponders placed inside a communication volume defined by theantenna, particularly by the geometrical dimensions thereof. By way ofexample, the communication volume is provided inside a cylinder or aparallelepiped rectangle around which the antenna is arranged.

In order to shield the antenna, particularly so that it does not disturbits environment, those skilled in the art know, in accordance with FIG.1, to arrange a central coil 2 defining inside its turns 4 acommunication volume 6 and, on either side of said coil 2, two shieldingcoils 8 and 9. In order not to decrease the communication volume of theantenna, coils 8 and 9 are arranged at a certain distance from coil 2.In fact, in order to cancel out the magnetic field outside the antenna,coils 8 and 9 are powered with a phase shift of 180° relative to thecentral communication coil. As appears in the lower graph of FIG. 1, asharp decrease in the field amplitude of the shielded antenna occursbetween the three regions dominated by the three respective fields ofthe three coils concerned. These magnetic field amplitude decreaseregions 11 and 12 thus result from the aforementioned 180° phase shiftfor powering the shielding coils. It will be noted that, in regions 11and 12, the magnetic field decrease is relatively large, such thatcommunication between the reader or transmitter and/or receiver andtransponders cannot be guaranteed in these regions. Consequently, theactive zone ZA of the sheathed antenna shown in FIG. 1 is limited toinside the geometrical dimensions of coil 2. This constitutes a majordrawback for such a device.

In fact, the sheathed antenna of the prior art according to FIG. 1 has auseful communication volume of relatively small length ZA in relation tothe total length L of the shielded antenna.

It is an object of the present invention to overcome the aforementionedmajor drawback by proposing a reader or transmitter and/or receiver witha sheathed antenna whose useful communication zone substantiallycorresponds to the total length of the shielded antenna.

The invention therefore concerns a reader or transmitter and/or receiverfor communication with transponders whose antenna is formed of severalturns defining a central axis and an overall internal volume,characterized in that the antenna includes a first group of turnsforming at least one first coil and a second group of turns forming atleast one second coil, these first and second coils being powered inphase quadrature and arranged to generate a communication field with anapproximately constant amplitude over substantially the entire length ofsaid antenna along its central axis and decreasing rapidly outside theantenna as it moves away from the latter.

In a particular embodiment, the shielded antenna of the prior art shownin FIG. 1 is altered by incorporating two compensation coils between thecentral coil and respectively the two end coils, these two compensationcoils being powered with a phase shift of 90° relative to the otherthree coils. These two compensation coils are also powered with a phaseshift of 180°, so as to quickly cancel out their resulting field outsidethe antenna, and are arranged relative to the first three coils shown inFIG. 1 so as to compensate for the decrease in the magnetic field inregions 11 and 12, i.e. between central coil 2 and end coils 8 and 9.

The present invention will be described in more detail with reference tothe annexed drawing, given by way of non-limiting example, in which:

FIG. 1, already described, shows a shielded antenna according to theprior art and the resulting amplitude of the magnetic field along itscentral axis;

FIG. 2 schematically shows a first embodiment of a communication readeror transmitter and/or receiver according to the invention with a graphgiving the amplitude of the magnetic fields present and the resultingmagnetic field;

FIG. 3 shows a particular variant of the first embodiment;

FIG. 4 shows an electric diagram of the powering of the antenna coils ofthe first embodiment, and

FIG. 5 schematically shows a second embodiment of a reader ortransmitter and/or receiver according to the invention, with a graphgiving the amplitude of the magnetic fields present and the resultingmagnetic field.

With reference to FIGS. 2 to 4, a first embodiment of the invention willbe described hereinafter. According to the invention, between centralcoil 6 and the two end coils 8 and 9, two other coils 16 and 17 arearranged, powered in phase quadrature in relation to coils 6, 8 and 9.More specifically, like in FIG. 1, coils 8 and 9 are powered by thepowering and control means 20 of reader 22 with a phase shift of 180°relative to central coil 6. Then, coils 16 and 17 are also powered witha phase shift of 180° in relation to the other and with a phase shift of90° in relation to the other coils 6, 8 and 9. Finally, the two coils 16and 17 are arranged such that their magnetic field along the centralaxis 24 of antenna 14 is maximum respectively in the two regions 11 and12 where the resulting magnetic field for the three coils 6, 8 and 9decreases or is cancelled out, as is shown in the graph of FIG. 2 whichshows the amplitude of magnetic field H along central axis 24 of theantenna. The supply phase shifts are described by a cosine (Cos) andsine (Sin) supply with one of the two arithmetic signs +/− placed infront. The position of each coil along central axis 24 and the featuresof each coil are determined so as to obtain a relatively constantamplitude of magnetic field 30 inside volume 32 defined by the antenna,i.e. by the set of coils defining a total length L on axis 24.

Owing to the features of the invention, the shielding of the mainantenna, i.e. central antenna 6, is arranged such that the overallvolume defined by the set of coils provided forms the useful volume forcommunication with transponders. In other words, the shielding isintegrated in the antenna itself. Inside each antenna there is nosignificant decrease in, or cancelling out of the magnetic field alongactive zone ZA, so that the reader according to the invention cancommunicate with any transponder located inside volume 32 defined by theset of coils forming the antenna. The arrangement of coils powered inphase quadrature compensates for the cancelling out of the magneticfield due to the counter-antennae powered with a phase shift of 180°.The vector sum of all the fields generated by the set of coilscorresponds to a quadratic sum between the resulting field of the firstgroup of coils 6, 8 and 9 and the resulting field of the second group ofcoils 16 and 17. Each coil is formed of at least one turn. Thus, thefirst group of coils forms a first group of turns whereas the secondgroup of coils forms a second group of turns.

It will also be noted that the two coils 16 and 17 are powered with aphase shift of 180° so as to ensure mutual shielding outside theantenna. The electric power supply diagram of the coils is given in FIG.4. In order to obtain the phase shift of 180° between coil 6 and coils 8and 9, and respectively between coils 16 and 17, the turns of each coilare wound in a first direction for coils 6 and 16 and in the otherdirection for coils 8, 9 and 17.

FIG. 3 shows a variant of the arrangement of an antenna according to theinvention. The position of the five coils 6, 8, 9, 16 and 17 is shownschematically in the top drawing. The central coil 6 includes 28 turnsand extends along axis 24 between −13.5 cm and 13.5 cm. Coils 8 and 9are each formed of 18 turns and are located respectively at −70 cm and+70 cm. When they are being powered, a current of 1 A passes throughthese three coils of the first group. The two coils 16 and 17 of thesecond group each include 15 turns and are respectively placed at −30 cmand +30 cm. The electric powering of this second group is a current of1.57 A.

On the bottom graph giving the amplitude of the magnetic field alongcentral axis 24, it will be observed that the total resulting field 36is substantially constant inside the antenna over the entire distancebetween the two end coils 8 and 9. This graph also shows on the onehand, the amplitude of magnetic field 38 generated by the first group ofcoils, and on the other hand, the amplitude of magnetic field 40generated by the second group of coils.

FIG. 5 shows schematically a second embodiment of the invention. In thetop part of this Figure, it will be noted that only four coils formantenna 42, namely a first group formed of coils 44 and 46 and a secondgroup formed of coils 48 and 50. The bottom graph of FIG. 5 showsamplitudes 52 and 54, respectively generated by the first and secondgroup of coils. The total resulting magnetic field is given by curve 56,which corresponds to the quadratic sum of curves 52 and 54.

As in the first embodiment, the coils of the second group are powered inphase quadrature relative to the coils of the first group. Moreover, thetwo coils of the same group are powered with a phase shift of 180° so asto generate mutual shielding. The resulting amplitude 56 inside thevolume defined by antenna 42 is substantially constant but has a slightvariation. Thus, this second embodiment saves one coil but has to becontent with a certain field variation inside the volume of the antenna,i.e. active communication zone ZA with the transponders. However, withinthe scope of the present invention, such a relatively small variationwith respect to the amplitude of magnetic field H can be consideredsubstantially constant.

By way of example, antenna 42 is arranged in the following manner: coil46 extends from −70 cm to −39 cm and coil 44 extends from −22 cm to 9cm. Coil 48 extends from −9 cm to 22 cm and coil 50 extends from 39 cmto 70 cm. All of the coils are formed of 15 turns and are powered by anelectric current of 1 A. The amplitude curves given in the graphcorrespond to this numerical example.

Of course, those skilled in the art could optimise the arrangement ofthe reader according to the invention, in particular of the coils of itsantenna to obtain the best result sought by the present invention,namely a substantially constant field inside the geometrical volume ofthe antenna so as to allow efficient communication between thetransponders placed inside the latter.

1. A reader or transmitter and/or receiver for communication withtransponders and including an antenna formed of a plurality of turnsdefining a central axis and an overall inner volume, wherein saidantenna includes a first group of turns forming at least one first coiland a second group of turns forming at least one second coil, said firstand second groups of turns being powered in phase quadrature andarranged so as to generate a total magnetic field with an approximatelyconstant amplitude over substantially the entire length (L) of saidantenna along its central axis and decreasing rapidly outside saidantenna as it moves away from the latter.
 2. The reader or transmitterand/or receiver according to claim 1, wherein said first group of turnsis formed of three coils comprising one central coil and two end coilsrespectively placed at the two ends of the antenna, said central coilbeing powered with a phase shift of 180° relative to the two end coils,wherein said second group of turns is formed of two compensation coilsarranged between said central coil and respectively the two end coils,so as to compensate for the decrease in, or cancelling out of themagnetic field between the central coil and the two end coils used forshielding said central coil, the two compensation coils being poweredwith a phase shift of 180°.
 3. The reader or transmitter and/or receiveraccording to claim 1, wherein said first group of turns if formed of twocoils powered with a phase shift of 180° and in that said second groupof turns is formed of two coils also powered with a phase shift of 180°,the two coils of the first group being placed at a certain distance fromeach other, the distance being substantially equal to that separatingthe two coils of said second group, said first and second groups beingpositioned relative to each other such that each of said groupscompensates for the decrease in, or cancelling out of the magnetic fieldbetween the two coils of the other group.
 4. The reader or transmitterand/or receiver according to claim 2, wherein the first group of turnsis powered with an electric current of a lower value than that of thecurrent flowing in the second group of turns, the number of turns ofeach coil being provided such that said total magnetic field issubstantially constant inside said overall inner volume of the antenna.5. The reader or transmitter and/or receiver according to claim 3,wherein one coil of the first group of turns is partially superposedonto one coil of the second group of turns.